ahenderl

My Cherokee starts and idles fine, however, many times after warming up, the idle jumps to about 1500rpm. I don't notice it while in gear, but when I stop and shift to neutral or park the rpms jump way up.

I assumed the problem was the IAC (idle air control) motor on the throttle body, so I replaced it with a new one, but the problem still persists.

About 2/3rd's of the time I get home or to work and shift to park, the engine rpm is too high. If I shut the motor off and restart the vehicle, it idles properly at about 800-900 rpm. Sometimes, just to test it, I shift to neutral at a traffic light and it will idle too high. Again, if I shut it off and restart it quicky, it will idle fine for while.

Any ideas? I assume that some sensor is directing the PCM to tell the IAC to increase the idle, but am not sure where to look. I read through the FSM, but did not get a clear answer.

Thanks in advance.

Abovetimberline

Have you checked your tps? It will cause the same issues that you are having.

Burns

mine has been doing this also....I cked the TPS and its plugged in right lol

ahenderl

I bought a TPS, but the one that came in had the wrong style of connector. I'll try that next.

rivercrazy

Check your vacuum lines. I know on mine if I pull a vacuum hose off the intake while parked the idle will jump up. Maybe you have a leak or a loose line?

ahenderl

Thanks...I'll check that. What has me perplexed is that it corrects itself if I shut it off and fire it right back up.

87CherokeeXJ

I have an '87 XJ that does the same thing. I've replaced both sections of the wiring harness, (excluding the part that runs to the injectors), replaced the IAC, replaced the ECU, replaced the ignition switch on the steering column, double checked all grounds and added another ground, as well as taken it to a mechanic and had the TPS adjusted and all vacuums checked and hooked up properly. It seems to do it after I have been driving it for a while. I'm stumped though.

rivercrazy

Not to change the subject, but in yur sig you said you've got ZJ discs in rear. Did you swap axles, hubs, or what? How hard was that?

swells

mine has done the same thing for the whole time I have had it. I have changed all the sensors all vacume lines, etc. when it does this I do a computer resset and it seems to fix the problem.

ahenderl

Not hard. No axle swap. ZJ discs will bolt to your axle, but the installation varies depending if you have a Dana 35 or Chrysler 8.25. The Dana 35 is a direct swap. The Chrysler 8.25 requires that you enlarge the hole in the center of the caliper bracket because the axle tube is a larger diameter.

How do you reset the computer? Mine never had this problem until about a month or so ago and we have owned it since new. I hate crap not to work right.

rivercrazy

Mines the C8.25. That sounds like a good project for down the road!

To reset your computer just disconnect your battery cables for about 5 min. You might need to touch the disconnected cables together for a min to discharge any juice in the system.

swells

to do a resset of the computer you, disconnect the possitive battery terminal, touch it to ground for 30 seconds,(some where on the engine), reconect the battery . turn the key on (but don't start) turn the headlights on , turn them back off, then turn the key off. this is how it is explained in the manual.

ahenderl

Turned out to be the TPS.

The 1st TPS I bought fixed the problem for a few days, but then the problem got worse. Since it had been replaced, I figured it couldn't be the TPS, right?...wrong. Initially I had a high idle, but the problem got worse and the trans wouldn't shift until very high RPM.

It finally threw a check engine light so I picked up an OBD2 scanner. It showed 2 TPS errors and a transmission code, too. It allowed me to monitor each sensor and my TPS would show 90%-100% throttle at idle. Even with the engine off, you could monitor the sensors if the key was set to run, so I removed the throttle body and checked the TPS reading removed from the throttle body. It was all over the place, but usually read 90%-100%.

The FSM says the TPS does somewhat effect transmission shift points. Mine was sending a full throttle response so my shift points were as though I had my foot on the floor...very frustrating.

Anyway, I bought another TPS and everything is back to normal.

Sorry for such a long post, but I know other people have had the same problem and thought this may help. Never assume that just because you replaced a part that it couldn't be bad out of the box.

KolbyMc

I have the same problem, I have replaced my map sensor, tps, block heater sensor, cleaned out my intake, butterfly valve, checked all my vacuum lines. But mine also sounds like it has a miss fire, and spits oil everywhere from high oil pressure, I just replaced my cold air intake filter and it's full of oil. All my seals are New. i don't know what else to do with it and all the techs around me have no clue.

damonfaulkner

where did you buy your obd2 scanner. hoe much?